1. Field of the Invention
The present invention relates generally to a surgical instrument, more particularly, pertains to a passive sensing reference electrode for an implantable pulse generator.
2. Description of the Prior Art
The delivery of an electrical stimulus to tissue induces a field which is generally orders of magnitude higher in amplitude than the field caused by the electrical activity of the tissue itself. When the stimulus ends, electrical fields remain in tissue due to two factors, the first being that electrochemical equilibrium at the electrode tissue interfaces which has been disturbed by the stimulus has to reestablish itself, and the second being that an output capacitor of the implantable pulse generator generally recharges through the same electrical circuits including the heart as through which it discharges.
When one wants to measure or sense the electrical activity of the tissue, it is generally necessary to supress or blank the sensing amplifier of the implantable pulse generator during a stimulus to avoid overloading. When the blanking is over and the sensing amplifier is reconnected to a lead connected to the implantable pulse generator forming a pacemaker, the sensing amplifier abruptly senses a different potential than was present at the time of initial blanking due to the electrode reaction and the recharging of the output capacitor which causes unwanted artifacts in the sensing signal.
FIG. 1 illustrates an electrical circuit schematic diagram of the prior art circuits showing the sense amplifier connected directly to the electrode and housing in a unipolar application, or directly to the electrodes in a bipolar application. As a consequence, when the stimulus ends, electrical fields remain in the tissue and are subsequently sensed by the sense amplifier as discussed. While the amplifier is usually suppressed or blanked during the stimulus to avoid overloading, the sensing amplifier senses a different potential than was present at the time of initial blanking when reconnected to the signal causing unwanted artifacts in the sensed signals.
The passive sensing reference electrode of the present invention provides an electrode which is positioned on the implantable pulse generator in such a way that electrical continuity with the tissue or body fluid is assured. This electrode is to be used as a reference electrode for sensing purposes only in contrast to the present situation where the surface of the pacemaker can of a unipolar implanted pulse generator or the electrode ring for a bipolar system serves as a reference or indifferent electrode for applying the stimulus current also. Ideally, a reference electrode for sensing purposes should never carry any current, as this leads to polarization and electrochemical instability at the site of the reference electrode. The proposed passive sensing reference electrode is such a reference for sensing only and its application allows for substantially shorter blanking time while avoiding the risk of improper sensing of stimulus afterpotentials.